Airscrew



Dec. 16, 1947,

, H. l.. MILNER AIRSCREW Filed Oct, 51, 1942 2 Sheets-Sheet 2 Patented Doc. i6, 1947 VuNireo srAras PATENT ortica" AIBSCREW Harry Lawiey Milner, Cheltenham, England, as-

signor to Rotol Limited, a British Company Application October 3l, 1942, Serial No. 464,086

In Great Britain November 5, 19,41 A

As claims. (o1. 17o-159)' In the manufacture of an airscrew, great care is taken to ensure "that it is accurately balanced so that when in .use it shall not give rise to ob- ,iectionable and dangerous vibration. Each blade in manufactured to the finest practicable tolerances of dimensions and weight, and is nally balanced against a standard blade, both horizontally and vertically, the latter in two different` pitch-settings 90 apart; iinally the blades are assembled in the hub so as tov be similarly situated thereon with a high degree of 'accuracy and in this way a satisfactory degree of dynamic balance can be achieved. It has been found, however, that some airscrews which satisfy all the usual tests give rise to dangerous vibrations when in use, and lt has not hitherto been possible to cure or avoid this trouble.

' The problem has become more serious since the advent of high-powered engines for which,

the airscrews are of largerdlameter, and'are more heavily loaded, and I have, therefore, in-

vestigated the question of the aerodynamicall 'balance of an airscrew.

The total aerodynamic loading on the blades of an airscrew can be resolved into components acting axially and in the plane of rotation respectively, and for perfect balance the effective sums of the axial and torque-plane components of all the blades should be separately zero.

Thus the axial loading or thrust exerted by the different blades of an airscrew ought to generateidentical thrust-moments about the axis since any variation in thrust-moment of the individual blades would give rise to a gyratory couple, thereby causing vibration.

For smooth operation the same equality should exist between the torque absorbed by the difierent blades of an airscrew. It-can be shown, however, that the conditions which produce balanced thrust-moments also satisfy, for all practical purposes, the necessary condition of equal turques. it is sufcient, therefore, to secure equality of the thrust-moments in the manner' 'ltultous differences in form and/or ldimensions of the blades, whereby the airscrew is aerodynarni cally balanced.

This invention also comprises a method of determining the relative pitch-settings of the blades of an airscrew, comprising the steps of: (a) measuring the diierences in form and/or dimensions from those of a master blade, (b) assessing the thrust-moment produced by such differences, (c) calculating the pitch-setting of the master blade which would produceA an equal thrust-moment and (d) applying such calculated pitch-setting as a correction to the setting of the f blade in question.

The master blade for the'purposes of the above method may be a drawing oi the blade as del signed, or it may be an actual'blade produced in as nearly perfect a form as is practicable, or it could in some instances be an arbitrarily selected blade, which may have fortuitou's errors in form orfdimensions, but which is used as a standard oi' comparison and matching for all other blades. Preferably, the second-mentioned" kind of master blade is used in practice, that is to say, as nearly as possible a perfect blade, so that it can be used for the dynamic balancing as well as the aerodynamic balancing.4

" According to another feature of this invention,

Y in the method above set forth, the first step of measuring the differences in fori'n and/or di-A mensions is effected by determining the angular position of the line of zero-lift relatively to a datum plane, at each of a number of stations along the blade, and its error as compared with that of a master-blade.

According to another feature of this invention, the line of zero-lift is determined for both the blade and the master-blade by the same empirical method, and preferably the master-blade yis a blade manufactured as nearly correct as is possible, and the line of zero-lift isV determined on both the master-blade and the blade in quesi tion by a gauge as described in the specification of my concurrent MESE/41.

In the accompanying drawings: Figure l. shows a blade-section; and Figure 2 is a developed View to a smaller scale of e, complete blade.

Figure 3 illustrates diagrammatlcally a threebiaded airscrew in accordance withthe present British patent application No.

` invention.

For the purposes of this invention, the thrustmoment of a blade can be determined in accordance with the simple blade-element theory. and

` thrust-moment is determined as it can be shown that the change in thrust-ino ment of an' element I0 of the'blade liisee Figure 2)'due to aideparture from the correct form or blade-angle, is approximately a function of Calama, where C, is a chord of the section, and a:

obtain 0. Its value will then be marked on each blade of the airscrew so that during Vthe final assembly the blade-anglesv at, say, 0.7 radius, instead of being made equal, as is the usual prac- 5 tice, areadlusted by the indicated amount A0.

is a radius of the section from the axis of rotation I2 ofthe airscrew, as shown on Figure 2, and a0 is the angular.l errorin the position of the zero-lift line. which the correct angular position of the zerolift line, or its position in a master-blade as indicated at I8, and the position as measured on the finished blade under test, is indicated at i4. These two lines intersect at the centre of curvature of the trailing edge of the blade section id and the angle 86 is the angle between them.

For the master-bladek as is assumed to be zero at all sections, but forthe blade under examination, the actual blade-angles may contain at various stations along the blade small errors 69 and the total change in thrust-moment due to these verrors becomesl a function of ECxaMx. It will be appreciated that these errors can be both positive and negative. By taking a number of measurements of a@ at different\sections along the blade, acurve of 033.50 with a: as abscissae can be plotted and its` area measured, this area representing the departure of the thrust-:nement from the ideal,

. since7 the measurements are taken from the basic Y rzero-lift lines.

Still using the same approximations, the pitchsetting of the master-blade which would give this El"Cxcadx Ao- EU'CJISI f If now this angle A0 be applied', withdue rregard to its sign,'as a correction to the setting of the bladeunder investigation, itwill give the same thrust-moment as the master-blade, and would,

therefore, in conjunction therewith be aerodynamically balanced.

The convenient formula given above is basedV on the simpleblade-element theory but it can 45 be shown that the same result, within 1%, is obtained by employing more exact airscrew theory. It will als'oibe appreciated that by using the s ame empirical method for determining the position ofth'e zero-lift line andthe same approximations in the assessment of the thrust-moment,

.any errors introduced thereby tend to vanish,

and the ilnal result has been found to `be of suiilciently high accuracy to provide aerodynamical balancing and smooth running of an airscrew which, when assembled in the usual manner, gave rise to dangerous vibration.

vSince the airscrew assembly according to this invention does not have the blades all mounted in exactly similar settings on the hub, there is thereby produced a change in the dynamic balance, but it is found that this change is negligible, and in any case it can, if desired, be restored by the usual methods ofv weighting the blade of the airscrew hub. I

A general manufacturing procedure, according to this invention, will be to determine the zero-lift line at a number of sections oi the blade and to compare them with` the corresponding design angles, or preferably with the corresponding angles on a blade which is manufactured as nearly correct as possible, and is used as a master-blade as a standard of comparison. The diiferences between the corresponding angles can then be applied in the manner above-described to This is indicated in Figure 1 in In this way, it can be assured that any blades, when assembled, will give an aerodynamically balanced airscrew. and if it is thought desirable the blades for an airscrew can be further l0 matched by using selected blades having the same or approximately the same corrections to be applied to them. l

They airscrew illustrated in Figure 3 comprises a h'bi having a` central opening 'il by which it is -mounted on its \driving shaft, and is provided with three sockets i8, i9, and which receive 'respectively blades 2|, 22, and 23. The root ends of the blades are cylindrical and are adjustable in their respective'sockets. On each socket there 20 is provided a datum mark 24, and the root end yof each blade has marked on it an index mark 25, 2B and 21 respectively. If a blade is found tohave the desired aerodynamic characteristics, its index mark is set in alignment with the datum zkmarkon the socket as shown in thecase of blade 2l. If the herein described method of assessing the aerodynamic characteristics of the blade shows that a correction must be applied to thisl setting, the index mark is displaced from the datum by the desired amount. yThe blade 22 is shown as adjusted in one direction, and the blade 23 `is shown as adjusted in the opposite direction, for positive'and negative corrections.

It will be appreciated thatthe pitch angles at 35, which the blades are thus set would not neces`V sarilybe different, since eachtis derived as the result of distinct measurement and computation.

An important advantage 'ofi this invention is that since it is possible tocorrect fortuitous er- 40 rors in the manufacture, the manufacturing tolerances can be considerably increased, thereby greatly facilitating the rate of production without impairing the emciency and satisfactory operation ofthe airscrews.

I claim:

Qualities of an airscrew blade as compared with those of a master blade, which comprises the i steps-of (a) measuring by a convenient empirical method the direction relatively to a fixed datum of the zero-lift line of the section of a master blade at a num-ber of stations along the blade,

(b) measuring by the same empirical method the zero-lift setting of the master blade which am would produce an equal thrust moment.

2. The herein described method of producing an aerodynamically balance-multi-bladed propeller havingr a hub in which the blades are adjustably mounted which comprises the steps of (a) measuring by a convenient empirical method the direction relatively Nto a fixed datum of the zero-lift line of the section of a master blade at a number of stations along the blade, (b) measuring by the samer empirical method any differences in direction of the corresponding lines of the blade being assessed, (c) determining the thrust-moment produced by such differences; (d) calculating the departure from the zero-lift setting of the master blade which would produce an equal thrust moment, said blade having a 1. A method of assessing the aerodynamic marking thereon showing immediately the cor-.-

rect pitch setting calculated from said steps, and assembling each said blade in the hub at an individual pitch angle which is the result of said calculations.

3. An airscrew comprising a hub and a plura1- ity of blades, set in said hub', each blade prepared in advance of setting in its hub by the following steps of (a) measuring lby a convenient immediately the correct pitch setting calculated from said steps, said blades being each assembled in said hub at an individual pitch angle which is the result of said calculations.

HARRY LAWLEY MILNER.

REFERENCESv CITED The following references are of record in the empirical method the direction relatively to a i@ me of this patent:

fixed datum of the zero-lift line of the section of a master blade at a number of'stations along` the blade (b) measuring by the same empirical method any diierences in direction oi the corresponding lines of the blade being assessed.

(c) determining the thrust-moment produced by such differences; (d) calculating the departure from the zero-lift setting oi the master blade which would produce an equal thrust moment.` said blade having a marking thereon showing UNITED STATES PATENTS 

